This invention relates to gas or vapor pumping systems, and more particularly to hybrid gas or vapor pumping systems including a rotary lobe or Roots pump as a first stage and a liquid ring pump as a second stage. The invention is especially of interest in connection with pumping systems for providing reduced pressure or "vacuum". For convenience herein, gases and vapors are referred to generically as gas.
Two-stage gas pumping systems having a Roots pump as a first stage and a liquid ring pump as a second stage are known as shown, for example, by Huse U.S. Pat. Nos. 3,642,384, 3,922,110, and 3,956,072. In such systems, the pressure differentials at which the rotary lobe pump can operate are primarily limited by the temperature differential across the pump. The components of a rotary lobe pump operate with close clearances, and the thermal expansion of these components must be controlled. At high vacuum this thermodynamic consideration becomes more acute because the less dense gas being pumped has less ability to transfer heat from the pump components.
Various techniques are known for limiting temperature rise through a rotary lobe pump. One of these techniques is the injection of a cooling liquid (e.g., water) into the inlet of the pump (see, for example, Huse U.S. Pat. Nos. 3,642,384 and 3,922,110). Although sometimes useful, this method can adversely affect available rotary lobe pump capacity due to the introduction of additional vapor load. Also, at higher vacuum levels, the vapor pressure of the injection liquid may become critical and therefore a limiting factor. For example, water generally cannot be used as the injection liquid when the inlet pressure drops below 20 mm HgA. In addition, the amount of liquid that can be injected is limited, and when water is used, there is the potential for plating of minerals on the lobes as the water passes through the rotary lobe pump.
Another known technique for limiting the temperature rise across a rotary lobe pump is so-called bypass cooling. In this technique a small amount of gas from the rotary lobe pump discharge is cooled and then re-introduced into the compression chamber of the pump. The principal disadvantage of this approach is the need for and additional expense of the gas cooler. The amount of cooling which can be provided in this way is also limited.
Still another known technique for limiting temperature rise across a rotary lobe pump is jacketed cooling, e.g., with cooling liquid or gas jackets surrounding the pumping chamber of the pump (see, for example, Higuchi et al. U.S. Pat. No. 4,789,314). This avoids the possible problems associated with cooling liquid injection. However, the ability of a cooling jacket to keep the lobes themselves cool is limited. Also, the addition of jackets can significantly increase the cost of the rotary lobe pump.
In view of the foregoing, it is an object of this invention to improve and simplify two-stage gas pumping systems having a rotary lobe pump as a first stage and a liquid ring pump as a second stage.
It is a more particular object of this invention to provide improved and simplified cooling for the rotary lobe pump which is the first stage in a gas pumping system having a liquid ring pump as the second stage.